1. Field of the Invention
The present invention relates to a CRT (cathode ray tube) protector circuit for protecting a CRT from an overcurrent or an overvoltage.
2. Description of the Related Art
A CRT for use in a television receiver or the like is provided with a CRT protector circuit on the high-voltage side, so as to protect the CRT itself from an overcurrent which may flow into the CRT or an overvoltage which may be applied to the CRT.
FIG. 1 shows a CRT protector circuit 10 in the related art. Referring to FIG. 1, a horizontal output circuit 12 is driven by a switching signal with a horizontal period at terminal 11. The horizontal output circuit 12 includes a horizontal output transistor Qa as a horizontal output element, and also includes a capacitor C and a damper diode D. The switching signal flows in a primary coil 15a of a flyback transformer (FBT) 15 to thereby allow a high-voltage current increased to a predetermined value to flow in a secondary coil (high-voltage coil) 15b of the flyback transformer 15.
The high-voltage current is rectified by a rectifier circuit 16 to obtain a high voltage HV. The high voltage HV is applied to an anode 21 of a CRT 20. A detector circuit 22 for detecting the high voltage HV is provided in an output stage of the rectifier circuit 16. The detector circuit 22 includes bleeder resistors 22a and 22b. A divided voltage OV obtained at a connection point p between the bleeder resistors 22a and 22b is supplied to a detector circuit 30 for detecting an overvoltage.
The detector circuit 30 functions also as a detector circuit for detecting an overcurrent which may flow through the anode 21 of the CRT 20. The current flowing in the CRT 20 is converted into a voltage OI by a resistor 25 connected to one end of the secondary coil 15b. The detection voltage OI obtained at a connection point q between the secondary coil 15b and the resistor 25 is supplied to the detector circuit 30. A control transistor Qb is controlled by an output from the detector circuit 30.
In this construction, when the high voltage HV is about 27 kV in a normal condition and it abnormally increases to 30 to 32 kV, the detection voltage OV also increases from a normal value to an abnormal value exceeding a reference value REF. Then, the detector circuit 30 detects this abnormal condition and turns on the control transistor Qb which stops the switching operation of the horizontal output transistor Qa. When the switching operation is thus stopped, the high voltage HV decreases to the normal value.
However, although the high voltage HV is returned to the normal value, the control transistor Qb holds in the on-state. Accordingly, once the abnormal condition is detected by the detector circuit 30, it is necessary to supply power again to restore the normal condition. If the abnormal condition is a permanent abnormal condition, it is necessary to change an abnormal element.
Also when an abnormal current (overcurrent) greater than the normal value flows into the CRT 20, the abnormal current is converted into the voltage OI by the resistor 25. If the voltage OI exceeds the reference value REF, the same control as that mentioned above is performed to prevent the breakdown of the CRT 20 due to the overcurrent. Also in this case, it is necessary to supply power again to restore the normal condition.
As described above, once the overvoltage or the overcurrent is detected in the above construction as shown in FIG. 1, it is necessary to supply power again to restore the normal condition. Accordingly, when the current is abnormally increased by an instantaneous internal discharge in the CRT 20 and the detector circuit 30 is operated, the image displaying operation of the television receiver remains stopped, in spite of the situation that the above discharge is stopped to restore a normal condition of the CRT 20.
Although the television receiver can be restored to the normal condition by supplying power again, the operation stop of the television receiver is apt to be misunderstood as a fatal fault by a user. That is, the user cannot determine whether the operation stop of the television receiver is a temporary protector operation or a fault such that any circuit element is necessary to exchange. Thus, the protector circuit shown in FIG. 1 is lacking in reliability.